An advance in moisturizing technology occurred with the development of emulsion technology and stable emulsions. An emulsion can be defined as a heterogeneous mixture that contains small droplets of immiscible or slightly miscible chemicals dispersed in another liquid. Emulsions contain lipophilic (oil soluble) or hydrophilic (water soluble) ingredients. An emulsion can be classified as oil in water (O/W) when oil droplets are stabilized by surfactants in a water continuous phase, or an emulsion can be water in oil (W/O) where the reverse is true, water droplets dispersed in a continuous liquid oil phase. Generally speaking, O/W emulsions are lower in viscosity than W/O emulsions. For moisturizers this manifests as a lotion (O/W emulsion) or a cream (W/O emulsion). Emulsifiers or surfactants are used to reduce the surface tension between the immiscible oil and water phases, with the application of shear droplets of one phase dispersed in the other.
The stability of the emulsion depends upon many variables. The size of the droplets is crucial; the smaller the droplet size, the more stable the emulsion. Formulators also use thickeners as a method to stabilize and change the esthetic feel.
Moisturizers are the most widely used cosmeceutical. Yet, the word itself is a misnomer. Moisturizers do not add water to the skin, even though the first or second ingredient on many moisturizer formulations is water. Most moisturizers function by placing a water impermeable film over the skin surface that decreases evaporation of water from the skin to the lower humidity atmosphere. Any water in a moisturizer formulation is a vehicle that evaporates, possibly drying the skin further, without enhancing water content. Water-based moisturizers may further damage xerotic skin, due to repeated wetting and drying of the skin surface.